Semiconductor wafer, polishing apparatus and method

ABSTRACT

A wafer polishing apparatus for polishing a semiconductor wafer. The polisher comprises a base ( 23 ), a turntable ( 27 ), a polishing pad ( 29 ) and a drive mechanism ( 45 ) for driven rotation of a polishing head ( 63 ). The polishing head is adapted to hold at least one wafer ( 35 ) for engaging a front surface of the wafer with a work surface of the polishing pad. A spherical bearing assembly ( 75 ) mounts the polishing head ( 63 ) on the drive mechanism for pivoting of the polishing head about a gimbal point (p) lying no higher than the work surface when the polishing head holds the wafer in engagement with the polishing pad. This pivoting allowing the plane of the front surface of the wafer to continuously align itself to equalize polishing pressure over the front surface of the wafer, while rotation of the polishing head is driven by the driving mechanism. This maintains the front surface and work surface in a continuously parallel relationship for more uniform polishing of a semiconductor wafer, particularly near the lateral edge of the wafer. A cassette of wafers and method of polishing are also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to apparatus for polishing semiconductor orsimilar type materials, and more specifically to such apparatus whichfacilitates equalization of the downward pressure over the polishedwafer surface and/or the polishing head of the apparatus.

Polishing an article to produce a surface which is highly reflective anddamage free has application in many fields. A particularly good finishis required when polishing an article such as a wafer of semiconductormaterial in preparation for printing circuits on the wafer by anelectron beam-lithographic or photolithographic process (hereinafter“lithography”). Flatness of the wafer surface on which circuits are tobe printed is critical in order to maintain resolution of the lines,which can be as thin as 0.13 microns (5.1 microinches) or less. The needfor a flat wafer surface, and in particular local flatness in discreteareas on the surface, is heightened when stepper lithographic processingis employed.

Flatness is quantified in terms of a global flatness variation parameter(for example, total thickness variation (“TTV”)) or in terms of a localsite flatness variation parameter (e.g., Site Total Indicated Reading(“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against areference plane of the wafer (e.g., Site Best Fit Reference Plane). STIRis the sum of the maximum positive and negative deviations of thesurface in a small area of the wafer from a reference plane, referred toas the “focal” plane. SFQR is a specific type of STIR measurement, asmeasured from the front side best fit reference plane. A more detaileddiscussion of the characterization of wafer flatness can be found in F.Shimura, Semiconductor Silicon Crystal Technology 191–195 (AcademicPress 1989). Presently, flatness parameters of the polish surfaces ofsingle side polished wafers are typically acceptable within a centralportion of most wafers, but the flatness parameters become unacceptablenear the edges of the wafers, as described below.

The construction of conventional polishing machines contributes tounacceptable flatness measurements near the wafer's edge. Polishingmachines typically include an annular polishing pad mounted on aturntable for driven rotation about a vertical axis passing through thecenter of the pad. The wafers are fixedly mounted on pressure platesabove the polishing pad and lowered into polishing engagement with therotating polishing pad. A polishing slurry, typically including chemicalpolishing agents and abrasive particles, is applied to the pad forgreater polishing interaction between the polishing pad and the wafer.

In order to achieve the degree of polishing needed, a substantial normalforce presses the wafers into engagement with the pad. The coefficientof friction between the pad and wafer creates a significant lateralforce on the wafer. This lateral force can give rise to certaindistortions in the polish, such as by creating a vertical component ofthe frictional force at the leading edge of a wafer. The verticalcomponent of the frictional force is created because the wafer ismounted to pivot about a gimbal point under influences of the lateralfriction forces. A change in the net vertical force applied to the waferlocally changes the polishing pressure and the polishing rate of thewafer, giving rise to distortions in the polish. Often the uneven forcescause the wafer's peripheral edge margin to be slightly thinner than themajority of the wafer, rendering the edge margin of the wafer unusablefor lithographic processing. This condition is a sub-species of the moregeneral problems associated with wafer flatness, and will be referred tohereinafter as edge roll-off.

Improvements in wafer polishers have helped reduce edge roll-off. Recentdesigns have incorporated conic bearing assemblies between the wafer andthe mechanism applying the polishing force while permitting freerotation of the wafer. Conic bearing assemblies are an improvement overtraditional ball and socket configurations because the gimbal point ofthe mechanism is at a point below the bearing, nearer the interfacebetween the wafer and the polishing pad. As the polishing pad rotatesbeneath the polishing head, friction between the pad and the wafercreate horizontal forces on the head, creating a moment on the head.This moment cants the polishing head with respect to the pad, applyinggreater force to the leading edge of the head. By lowering the pivotpoint of the polishing head toward a work surface of the polishing pad,or slightly below the surface, the torque moment applied to thepolishing head by frictional forces is either minimized, eliminated orimparted in a more desirable direction. Control of this moment resultsin more uniform polishing pressure at all points on the wafer and inmore uniform wear of the polishing pad. Wafers polished with a gimbalpoint near the work surface exhibit superior flatness characteristics,particularly near the outer edge of the wafer where conventionalpolishing processes exhibit characteristic “roll-off” and near thecenter of the wafer where slurry starvation may occur. Roll-off occursin polishers having a gimbal point above the work surface where thetorque on the polishing head due to friction presses the leading edge ofthe polishing head, and the wafer, into the polishing pad. Slurrystarvation occurs when the leading edge of the wafer and head press intothe polishing pad, pushing the slurry forward and inhibiting the slurryfrom flowing between the pad and the wafer. Despite these improvementsin the prior art, the edge of the wafer may still exhibit unacceptableroll-off and the center of the wafer may be insufficiently polished.

Controlling wafer rotation while lowering the gimbal point to at orbelow the work surface is more desirable, because controlling the gimbalpoint of the mechanism and the rotational speed of both the polishingpad and the wafer allows more control over the wafer polishing process.Freely rotating polishing heads, in contrast, provide little controlover the polishing process, as the polishing head and wafer simplyrotate in response to frictional forces between the wafer and thepolishing pad. Frictional forces can change between wafers and from onepolishing machine to the next (due to turntable and drive mechanismmisalignment, for instance), varying the rotational speed of thepolishing head and the characteristics of the wafer polish. This processcan lead to uneven polishing between wafers and cause increaseddegradation of the interior of the polishing pad. Since a freelyrotating wafer will tend to rotate at a faster rate, the inside of thepolishing pad sees more linear feet of wafer, wearing the pad morequickly near the pad's center. When the pad wears more quickly near thecenter, wafer flatness degrades because the pad is no longer flat. Ifthe rotational speed of the wafer is decreased, polishing quality isgreatly improved due to more uniform wear across the polishing pad.Moreover, pad wear impacts any “dishing” or “doming” of the wafersurface, which can be more effectively controlled by the rotationalspeed of the wafer. Thus, an improved design is needed incorporatingfurther features, such as a low gimbal point and wafer rotation control,for inhibiting edge roll-off and improving wafer flatness generally.

SUMMARY OF THE INVENTION

Among the several objects and features of the present invention may benoted the provision of a semiconductor wafer, semiconductor waferpolishing apparatus and method which improves the flatness of the wafersprocessed; the provision of such a wafer, apparatus and method whichreduces wafer edge roll-off; the provision of such a wafer, apparatusand method which increases the area of the wafer usable for lithographicprocessing; and the provision of such a wafer, apparatus and methodwhich improves site to site consistency between the outer ring sites andthe inner ring sites on the wafer.

Generally, a wafer polishing apparatus of the present inventioncomprises a base for supporting elements of the polishing apparatus. Aturntable having a polishing pad thereon mounts on the base for rotationof the turntable and polishing pad relative to the base about an axisperpendicular to the turntable and polishing pad. The polishing padincludes a work surface engageable with a front surface of a wafer forpolishing the front surface of the wafer. A drive mechanism mounts onthe base for imparting rotational motion about an axis substantiallyparallel to the axis of the turntable. A polishing head connected to thedrive mechanism for driven rotation of the polishing head is adapted tohold at least one wafer for engaging a front surface of the wafer withthe work surface of the polishing pad. A spherical bearing assemblymounts the polishing head on the drive mechanism for pivoting of thepolishing head about a gimbal point lying no higher than the interfaceof the front surface of the wafer and the work surface when thepolishing head holds the wafer in engagement with the polishing pad.This pivoting allows the plane of the front surface of the wafer tocontinuously align itself to equalize polishing pressure over the frontsurface of the wafer, while rotation of the polishing head is driven bythe driving mechanism. This maintains the front surface and work surfacein a continuously parallel relationship for more uniform polishing of asemiconductor wafer.

In another aspect of the present invention, a method of polishing asemiconductor wafer generally comprises placing the semiconductor waferin a polishing head of a wafer polishing apparatus and driving rotationof a polishing pad on a turntable of the polishing apparatus about afirst axis. Rotation of the polishing head is driven generally about asecond axis non-coincident with the first axis. The wafer held by thepolishing head is positioned so that a front surface of the waferengages a work surface of the polishing pad and is urged against thepolishing pad. The polishing head is held for free pivoting movementabout a gimbal point located no higher than the interface of the worksurface and the front surface of the wafer, as rotation of the polishinghead continues to be driven, so that the plane of the front surface ofthe wafer can equalize polishing pressure over the front surface of thewafer of the polishing pad in response to a net force about the gimbalpoint acting in a direction perpendicular to the front surface of thewafer, while preventing pivoting of the front surface of the wafer underforces parallel to the front surface of the wafer passing generallythrough the gimbal point. The wafer is disengaged from the turntable andthe wafer is removed from the polishing head.

In a final aspect of the present invention, a cassette of single sidepolished, monocrystalline semiconductor wafers is disclosed. The waferseach comprise a central axis and a front surface generally perpendicularto the central axis and polished to a finish polish. The wafers furthercomprise a back surface which is not polished to a finish polish and acircumferential edge. The front surface is uniformly flat for use inlithographic imprinting of circuits thereon in an area from the centralaxis at least to within 2 millimeters (0.08 inches) of thecircumferential edge. The wafers are not selected according to theirflatness.

Other objects and features of the present invention will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation of a conventional wafer polishingapparatus;

FIG. 1A is a schematic side elevation of the wafer polishing apparatusof FIG. 1 inside a non-contamination booth;

FIG. 1B is a schematic side elevation and partial section of the waferpolishing apparatus of the present invention;

FIG. 2 is an enlarged, fragmentary schematic of the wafer polishingapparatus showing a polishing head thereof in section;

FIG. 2A is graph depicting a comparison of the total linear distance thewafer travels over each point on the polishing pad for differentpolishing head rotational speeds;

FIG. 3 is an enlarged, fragmentary section of a second embodiment of thepolishing head of the present invention;

FIG. 4 is an enlarged, fragmentary section of a third embodiment of thepolishing head of the present invention;

FIG. 4A is a perspective view of a wafer carrier;

FIG. 5 is a schematic of a 200 millimeter (7.9 inches) diameter waferdivided into sites;

FIG. 6 is a graph depicting the largest SFQR value for any partial siteon each wafer of a set of wafers polished on a conventional waferpolisher;

FIG. 7 is a graph depicting the largest SFQR value for any partial siteon each wafer of a set of wafers polished on a wafer polisher of thepresent invention;

FIG. 8 is a graph depicting the average of the SFQR values for allpartial sites on each wafer of the set polished on a conventional waferpolisher;

FIG. 9 is a graph depicting the average of the SFQR values for allpartial sites on each wafer of the set polished on a wafer polisher ofthe present invention;

FIG. 10 is a schematic of a 200 millimeter (7.9 inches) diameter waferindicating movement of a lithography apparatus from focusing whole sitesto non-focusing partial sites;

FIG. 11 is a graph depicting the difference between an average of theSFQR values for each site of an outer ring of partial sites and anaverage of the SFQR values for each site of an immediately adjacentinner ring of whole sites for each wafer polished on a conventionalwafer polisher;

FIG. 12 is a graph depicting the difference between an average of theSFQR values for each site of an outer ring of partial sites and anaverage of the SFQR values for each site of an immediately adjacentinner ring of whole sites for each wafer polished on a wafer polisher ofthe present invention;

FIG. 13 is a graph depicting the percentile difference between anaverage of the SFQR values for each site of an outer ring of partialsites and an average of the SFQR values for each site of an immediatelyadjacent inner ring of whole sites for each wafer polished on aconventional wafer polisher;

FIG. 14 is a graph depicting the percentile difference between anaverage of the SFQR values for each site of an outer ring of partialsites and an average of the SFQR values for each site of an immediatelyadjacent inner ring of whole sites for each wafer polished on a waferpolisher of the present invention;

FIG. 15 is a graph depicting the percentile difference between themaximum SFQR value for any partial site of each wafer and the maximumSFQR value for any whole site of each wafer polished on a conventionalwafer polisher; and

FIG. 16 is a graph depicting the percentile difference between themaximum SFQR value for any partial site of each wafer and the maximumSFQR value for any whole site of each wafer polished on a wafer polisherof the present invention.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures, and specifically FIG. 1, a schematic of aconventional wafer polishing apparatus, generally indicated at 15,includes a mounting shaft 16, a polishing head 17, a wafer 18 and apolishing pad 19. The shaft 16, polishing head 17 and wafer 18 rotateabout a vertical axis, as the wafer is pressed into the polishing pad 19to polish the wafer. As will be discussed in greater detail below, thepolishing head 17 must pivot with respect to the shaft 16, so that thewafer 18 may remain in flatwise engagement with the polishing pad 19.The polishing head 17 and wafer 18 are mounted to pivot with respect tothe shaft 16 about a gimbal point P. In many conventional polishers,including the schematic of FIG. 1, the gimbal point P is located wellabove the interface of the wafer 18 and the polishing pad 19. Thedistance from the pad 19 to the gimbal point P is often as large aseveral inches, such as the two inch distance depicted in FIG. 1.

Turning to the present invention, specifically to FIGS. 1A and 1B, awafer polishing apparatus, generally indicated at 21, constructedaccording to the present invention is shown having a base, generallyindicated at 23, for housing and-supporting other elements of thepolishing apparatus. The base 23 may be of various configurations, butpreferably is formed to provide a stable support for the polishingapparatus 21. In the preferred embodiment, a booth 25 encloses the waferpolishing apparatus 21 and inhibit airborne contaminants from enteringthe booth and contaminating the apparatus and articles to be polished.Except as pointed out hereinafter with regard to the way the wafer isheld and polished by the polishing apparatus during polishing, theconstruction of the polishing apparatus is conventional. An example ofsuch a conventional single-sided polishing apparatus of the typediscussed herein is the Strasbaugh Model 6DZ, available from StrasbaughInc. of San Luis Obispo, Calif.

A turntable 27 is mounted on the base 23 for rotation with respect tothe base. The turntable 27 is circular and has a polishing pad 29mounted thereon for polishing a semiconductor wafer 35. The polishingpad 29 is preferably adhesive-backed for securing the pad to theturntable 27. The turntable and polishing pad 29 rotate conjointlyrelative to the base 23 about an axis A perpendicular to the turntableand polishing pad. The opposite side of the polishing pad comprises awork surface 37 engageable with a front surface 39 of the semiconductorwafer 35. During polishing, the polishing pad 29 is designed to receivea continuous supply of polishing slurry. The polishing slurry isdelivered to the pad 29 via a slurry delivery system (not shown).Polishing pads 29, polishing slurry, and slurry delivery systems arewell known in the relevant art. The rotation of the turntable 27 iscontrolled by a turntable motor and turntable control device (notshown). The turntable control device controls the rotational speed ofthe turntable 27 to further adjust the polishing of the wafer 35, aswill be discussed in greater detail below. The turntable control deviceand motor are well known in the relevant art.

A drive mechanism, generally indicated at 45, is mounted on the base 23above the turntable 27 for imparting rotational motion of the drivemechanism about an axis B substantially parallel to axis A of theturntable (FIG. 1B). The drive mechanism 45 comprises a motor 47 and agearbox 49 housed in a movable arm 53. The movable arm 53 pivots bothlaterally and vertically, so that the arm can pick up, polish andrelease the semiconductor wafer 35, as will be described in greaterdetail below. The drive mechanism 45 also includes a control device (notshown) for controlling the rotational speed of the drive mechanism toenhance the polishing characteristics of the polishing process. Themotor 47 is oriented horizontally within the arm 53 and connected to thegearbox 49, which comprises a suitable worm gear assembly (not shown),for converting the rotation of the motor about a horizontal axis intorotation of an output shaft 55 about axis B. The conversion ofrotational motor 47 energy in a gearbox 49 is well understood in the artand will not be further described here. The output shaft 55 passes fromthe gearbox 49 down through a double-row radial bearing 57 forcontrolling shaft orientation.

The base 23, booth 25, turntable 27, and drive mechanism 45 are eachwell known in the art and comprise the basic elements of the single-sidewafer polishing apparatus 21 noted above. The subject of the presentinvention is a new and useful improvement to such a polishing apparatus21. Turning to the new and novel features of the present embodiment, thewafer polishing apparatus 21 further comprises a polishing head,generally indicated at 63, pivotably and rotatably connected to thedrive mechanism 45 for driven rotation of the polishing head (FIG. 1B).The polishing head's 63 primary purpose is holding the wafer 35 securelyduring polishing so that the wafer may be polished evenly. The polishinghead 63 mounts on the lower end of the output shaft 55 so that theyrotate conjointly. Polishing heads 63 are conventionally used to performsingle-side polishing, but suffer various drawbacks relating to thequality of the polished wafer 35. The polishing head 63 of the presentembodiment avoids those drawbacks by further comprising a sphericalbearing assembly, generally indicated at 75. The assembly comprises anupper bearing member 77, a lower bearing member 79 and a plurality ofball bearings 81. The upper bearing member 77 and lower bearing member79 are not rigidly connected to one another and may move with respect toone another. The ball bearings 81 are engageable with the upper bearingmember 77 and the lower bearing member 79 for relative movement betweenthe members, so that the polishing head 63 may pivot relative to thedrive mechanism 45. The bearings 81 are preferably held within aconventional bearing race (not shown), as is well understood in theprior art, for holding the bearings in position between the bearingmembers 77, 79. The upper bearing member 77 is rigidly mounted on thedrive mechanism 45 while the lower bearing member 79 is rigidly mountedto the polishing head 63. The upper bearing member 77 and the lowerbearing member 79 have spherically shaped bearing surfaces arranged sothat the center of curvature of each spherical bearing surfacecorresponds to a gimbal point P. Any line normal to either bearingsurface passes generally through the gimbal point P, the pivoting centerof the assembly 75. Thus, the drive mechanism 45 and the polishing head63 also pivot about the gimbal point P. In the preferred embodiment, thebearing members 77, 79 and ball bearings 81 are formed from hardenedsteel or other material capable of withstanding repeated pivotingmotions of the polishing head 63 as it rotates. The surfaces are highlypolished to inhibit wear debris generation and to minimize frictionwithin the spherical bearing assembly 75 and create a highly smoothpivoting movement of the bearing assembly.

The arm 53 applies downward pressure to the polishing head 63 duringwafer polishing (FIG. 1B). As stated previously, the arm 53 pivotsvertically about a horizontal axis near the proximal end of the arm (notshown). A hydraulic or pneumatic actuation system is commonly used toarticulate the polisher arm 53, although other articulation systems arecontemplated as within the scope of the present invention. These systemsare well known in the relevant art and will not be described in detailhere. Downward force from the actuation system is transferred to thewafer 35 through the output shaft 55, the upper bearing member 77, theball bearings 81, and the lower bearing member 79.

The wafer polishing apparatus 21 further comprises a semi-rigidconnection, generally indicated at 89, between the drive mechanism 45and the polishing head 63 for imparting a rotational force from thedrive mechanism to the polishing head (FIG. 1B). The semi-rigidconnection 89 ensures that the polishing head 63 and drive mechanism 45rotate conjointly so the control device can regulate the speed of thedrive mechanism, and thereby the rotation of the wafer 35. Without thesemi-rigid connection 89, the upper bearing member 77 would rotate withthe drive mechanism 45 while the lower bearing member 79 and wafer 35would fail to rotate beneath the spherical bearing assembly 75. Theconnection between the drive mechanism 45 and the polishing head 63 mustbe semi-rigid so that the universal pivoting motion of the polishinghead with respect to the drive mechanism about the spherical bearingassembly 75 is unaffected by the driving force of the drive mechanism.The semi-rigid connection 89 is a flexible connection, which in thefirst embodiment is a torque transmittal boot 93 attached to the drivemechanism 45 and the polishing head 63. The boot 93 allows the polishinghead 63 to pivot with respect to the drive mechanism 45 about horizontalaxes passing through the gimbal point P of the spherical bearingassembly 75 for transmitting the rotation from the drive mechanism tothe polishing head. A ring 95 fits over the outer edge of the torquetransmittal boot 93 to secure the boot to the polishing head 63. Thering 95 and boot 93 each contain a plurality of matching holes 97 sothat a plurality of bolts 103 can pass through the ring and boot tofirmly hold the boot to the polishing head 63. The ring 95 reenforcesthe boot 93 so that the rotational force transmitted through the bootspreads evenly over the circumference of the boot. In the preferredembodiment, the torque transmittal boot 93 is made of an elastomericmaterial, such as rubber (e.g., urethane), having a stiffness capable oftransmitting the rotational energy of the drive mechanism 45 to thepolishing head 63 and a resiliency capable of allowing pivoting movementof the polishing head. Other materials capable of transmitting therotation energy and allowing pivoting motion of the polishing head 63are also contemplated as within the scope of the present invention.

The polishing head 63 is further adapted to hold the wafer 35 forengaging the front surface 39 of the wafer with the work surface 37 ofthe polishing pad 29 (FIG. 1B). The head 63 includes a lower body 109mounted on the lower bearing member 79. The lower body 109 rotatesconjointly with the lower bearing member 79 and rigidly connects to thetorque transmittal boot 93 as described above. Therefore, the boot 93transfers the rotational energy of the output shaft 55 directly to thelower body 109 of the polishing head 63. The lower body 109 additionallyincludes an inwardly directed annular flange 111 which projects inwardabove the upper bearing member 77 so that when the arm 53 lifts thepolishing head 63 upward, the weight of the lower body 109, a polishingblock 115 and the wafer 35 rest upon the rigid upper bearing member,rather than the torque transmittal boot 93. This flange 111 helpspreserve the torque transmittal boot 93 by not subjecting it to arepeated vertical tensile load when the arm 53 lifts the drive mechanism45 and polishing head 63. The lower body 109 further comprises aretaining ring 117 and mounting shim 119 mounted beneath the lower body,cooperating to create a seat for the polishing block 115 to mount on thepolishing head 63. The retaining ring 117 extends downward from theperimeter of the lower body 109 to provide lateral support for thepolishing block 115, and the mounting shim 119 is a flat annular ringwhich mounts on the underside of the lower body to separate the blockfrom the lower body. The polishing block 115 is a thick, rigid blockused as support for the wafer 35 during polishing. Polishing blocks 115are selected for their flatness and rigidity and are typically formedfrom ceramic materials due to their structural rigidity and temperaturestability. The wafer 35 is mounted on the bottom of the polishing block115 in a conventional manner by applying a wax layer to the polishingblock and adhering the wafer to the block, leaving the front surface 39of the wafer exposed and facing downward. The polishing block 115 isthen mounted on the lower body 109 by evacuating a cavity 125 formedbetween the lower body, shim 119 and polishing block. Evacuating thiscavity 125 holds the polishing block 115 securely on the polishing head63.

In operation, referring now to FIG. 2, the interaction of the polishinghead 63 pivotably mounted on the drive mechanism 45 is depictedschematically. Arrow D indicates the direction of movement of theturntable 27 with respect to the wafer 35. As discussed previously, thegimbal point P is the pivoting point of the entire spherical bearingassembly 75. The location of this gimbal point P with respect to thewafer 35 impacts the polishing characteristics of the polishingapparatus 21. As the polishing pad 29 rotates beneath the polishing head63, friction between the pad and the wafer 35 creates horizontal forceson the head, resulting in a moment on the head. By lowering the gimbalpoint P of the polishing head 63 toward the work surface 37, or slightlybelow the surface as shown in an exaggerated position in FIG. 2, themoment applied to the polishing head by frictional forces is eitherminimized or imparted in a more desirable direction. Control of thismoment results in more uniform polishing pressure at all points on thewafer 35 and in more uniform wear of the polishing pad 29. Wafers 35polished with a gimbal point P near or slightly below the work surface37 exhibit superior flatness characteristics, particularly near an outeredge 129 of the wafer where conventional polishing processes exhibitcharacteristic “roll-off.” Roll-off occurs in polishers having a gimbalpoint P′ above the work surface 37 where the torque on the polishinghead 63 due to friction presses the leading edge 131 of the wafer 35into the polishing pad 29. Because the wafer 35 is rotating, the leadingedge 131 of the wafer is constantly changing, creating a downwardlysloping edge, or roll-off, about the circumference of the wafer. Wherethe gimbal point P lies at the polishing interface, the moment decreasesbecause the friction forces pass through or very near the gimbal pointP. The leading edge 131 of the wafer 35 (or a retaining ring holding thewafer as discussed below) does not press the wafer 35 into the polishingpad 29 with as much force, decreasing wafer roll-off. In addition, lessof the polishing slurry is pushed forward of the wafer 35 and squeezedoff the pad 29 as the leading edge 131 of the wafer 35 moves across thepolishing pad, as compared with typical polishers having a gimbal pointP′ further above the work surface 37. With more slurry flowing towardthe wafer's 35 center, the center is subject to more polishing, furtherlessening the overpolishing of the wafer edge 129. Where the pivot pointP is below the work surface 37, the moment reverses, biasing thepolishing pressure to a trailing edge 133 of the head 63, furtherenhancing the amount of slurry able to flow beneath the wafer 35 andimproving polishing of the central portion of the wafer.

In the present invention, the gimbal point P lies near the work surface37 when the polishing head 63 holds the wafer 35 in engagement with thepolishing pad 29. This location allows the wafer 35 to continuouslyalign itself to equalize polishing pressure over the front surface 39 ofthe wafer, while the polishing head 63 is driven to rotate by thedriving mechanism 45. Because of the pivoting motion of the polishinghead 63, the front surface 39 is maintained in flatwise engagement witha work surface 37 for more uniform polishing of a semiconductor wafer35. Moreover, by pivoting about a point P lying at the polishinginterface, moments on the head 63 arising from friction forces directedparallel to the front surface 39 of the wafer 35 are virtuallyeliminated. In the preferred embodiment, the gimbal point P lies nohigher than an interface of the wafer 35 and the work surface 37 on aside of the interface containing the turntable 27. This configurationmaintains the work surface 37 and the front surface 39 in a nearlyparallel relationship by equalizing polishing pressure over the frontsurface for more uniform polishing of the wafer 35. This configurationfurther inhibits pressure points from forming near the leading edge 131of the wafer 35 due to pivoting of the head relative to the turntable27. Because the moment on the polishing head 63 applies slightly morepressure to the trailing edge 133 of the wafer 35, an adequate amount ofslurry can pass between the wafer and polishing pad 29 to improve waferpolishing.

The axis of rotation of the polishing head (axis B) is spaced apart froman axis of rotation (axis A) of the turntable (FIG. 1B). This helpsensure that the wafer 35 is subject to even polishing over a substantialportion of the polishing pad 29. The polishing pad is preferably muchwider than the wafer 35 and polishing head 63, so that no portion of thewafer passes over the central portion of the polishing pad duringpolishing. This helps increase the longevity of the polishing pad 29 andthe evenness of the wafer polish, because the wafer 35 interacts with amajority of the polishing pad.

Additionally, the polishing head 63 and the turntable 27 rotate atdifferent relative rotational speeds for more uniform and efficientpolishing of the wafer 35. Regulating the rotational speed of thepolishing head 63 impacts the wear pattern of the polishing pad 29,which in turn impacts wafer 35 flatness and polishing pad life. Therotation of the wafer 35 and the polishing pad 29 can be modeledmathematically to compare the relative velocities of each fordetermining what relative velocities will likely provide the most evenpolishing and longest pad life. FIG. 2A is a graphical depiction of theresults of such a comparison. The set of curves on FIG. 2A depict thetotal linear distance the wafer 35 travels over each point on thepolishing pad 29. Each curve represents a different rotational speed(Ω_(h)) of the polishing head 63, while the rotational speed of thepolishing pad 29 is held at a constant 200 revolutions per minute (rpm).For example, where the polishing pad 29 and polishing head 63 rotate atthe same rotational speed, 200 rpm (Ω_(h)=200 rpm), any point on thepolishing pad lying 60 millimeters (2.4 inches) from the center of thepad sees approximately 235 millimeters (9.25 inches) of wafer 35 passover that point during each revolution of the polishing pad. Tracing thecurve corresponding to a polishing head speed of 200 rpm (Ω_(h)=200rpm), FIG. 2A demonstrates that where the polishing pad 29 and polishinghead rotate at the same speed, the radially inner portions of thepolishing pad see more linear distance of wafer 35 pass over them thanthe outer portions of the polishing pad. Over time, this could lead togreater polishing pad 29 wear near the inner portion of the polishingpad 29. Ideally, each point on the polishing pad 29 should see anidentical amount of wafer 35 pass over during a single revolution. Butit is apparent from FIG. 2A that no combination of angular velocitieswould produce such a resulting horizontal line. The best availableprofile would distribute the wafer 35 distance seen by each portion ofthe pad 29 more evenly over the whole polishing pad. The curve where thepolishing head 63 rotates at a speed of 100 rpm (Ω_(h)=100 rpm) nearlyapproximates such a result. Therefore, rotating the polishing head 63near about 100 rpm often yields more even polishing of the wafer 35 andmore consistent wear over the polishing pad 29, because pad wear may beinferred from linear wafer distance seen by the polishing pad. Becausethese results are based upon relative velocities, they are scalable andthe velocity of the polishing head 63 may be expressed as a percentageof the rotational velocity of the polishing pad 29.

As discussed above, in the preferred embodiment, the polishing head 63is driven at a rotational speed less that the turntable 27. Were thewafer 35 and polishing head 63 allowed to freely rotate, they wouldrotate at approximately the same speed as the polishing pad 29, leadingto uneven wear of the pad. Thus, the drive mechanism 45 actuallythrottles the rotational speed of the polishing head 63 so that thepolishing head rotates at a rotational speed of between about fortypercent (40%) and about seventy percent (70%) of the rotational speed ofthe turntable 27. In the example above, this corresponds to an Ω_(h) ofbetween 80 rpm and 140 rpm. Based upon further experimentation and theabove analysis, this range has been found to be the optimal range forwafer polishing, producing more uniform polishing across the frontsurface 39 and more even polishing pad 29 wear. More particularly, thebest polishing is achieved where the drive mechanism 45 rotates at arotational speed of about fifty-five percent (55%) of the rotationalspeed of the turntable 27. In the example of FIG. 2A, this correspondsto an Ω_(h) of approximately 110 rpm.

Turning to a second embodiment of the polishing head of the presentinvention, a polishing head 153 connects to the drive mechanism 45 fordriven rotation of the polishing head (FIG. 3). The polishing head 153is adapted to hold a wafer 35 for engaging a front surface 39 of thewafer with a work surface 37 of the polishing pad 29. The polishing head153 is attached to the drive mechanism 45 via a spherical bearingassembly, generally indicated at 159, for pivoting of the polishing headabout a gimbal point lying near the work surface 37. The polishing head153 holds the front surface 39 of the wafer 35 in engagement with thepolishing pad 29, thereby polishing the wafer and allowing the plane ofthe front surface to continuously align itself to equalize polishingpressure over the front surface of the wafer for more uniform polishingof a semiconductor wafer.

A semi-rigid connection, generally indicated at 163, attaches to thedrive mechanism 45 and the polishing head 153 for transferring arotational force from the drive mechanism to the polishing head, whilepermitting universal pivoting motion of the polishing head with respectto the drive mechanism about the spherical bearing assembly 159. In manyfacets, therefore, the second embodiment is similar to the first.

Although similar, the second embodiment of the polishing head 153retains the wafer 35, imparts pressure on the wafer and transmitsrotation to the polishing head in novel ways. A membrane 169 is mountedon the underside of the polishing head 153 (FIG. 3). In the preferredembodiment, the membrane 169 is formed from silicone, although othersuitable materials are contemplated as within the scope of the presentinvention. The membrane 169 has an outer surface 171 engageable with thewafer 35 for mounting the wafer on the polishing head 153 and an innersurface 173 opposite the outer surface facing the polishing head. Thepolishing head 153 further comprises a ring-shaped retainer 177 thatencircles the membrane 169 and attaches to the polishing head to retainthe membrane on the head. The retainer 177 seals the periphery of themembrane 169 to the polishing head 153, while allowing the portion ofthe membrane not directly engaging the retainer to move independentlyinward and outward from the head a short distance. A cavity 179 definedbetween the membrane 169 and the head 153 is in fluid communication witha vacuum source. The vacuum is transmitted to the polishing head 153 bypassing through a series of channels 181 in an output shaft 55 and head.The membrane 169 has a hole formed therein so that when a vacuum isdrawn in the cavity 179, the membrane 169 can draw the wafer 35 upagainst the membrane and hold the wafer. The membrane 169 further holdsthe wafer by selectively varying air pressure within the cavity 179 forpressing the front surface 39 uniformly against the work surface 37.Although the second embodiment is capable of performing substantiallyidentical polishing as the first embodiment, the second embodiment isideally suited for polishing a wafer 35 previously polished on adouble-side polished wafer polisher. Such a wafer 35 is already polishedsubstantially flat, so that any additional polishing is aimed atremoving a uniform layer of silicon material over the entirety of thewafer, without generally impacting wafer flatness. The membrane 169 isparticularly well suited for such a purpose, as the retainer 177 ispressed firmly against the polishing pad 29 for retaining the wafer 35while the membrane allows the wafer to conform to the polishing pad forremoval of a uniform layer of silicon.

The spherical bearing assembly 159 further comprises an upper conicalseat 187 attached to and rotating with the drive mechanism 45 (FIG. 3).A lower spherical pivot 189 rigidly mounts on the polishing head 153 andextends upward toward the drive mechanism 45. The lower spherical pivot189 is engageable with the upper conical seat 187 for pivotable movementof the polishing head 153 with respect to the drive mechanism 45. Thelower spherical pivot 189 has an upwardly directed spherical face 191.Any line normal to the spherical face 191 passes through the gimbalpoint of the pivot. Although the construction of the spherical bearingassembly 159 is substantially different than the first embodiment, thepivoting motion created is substantially similar, resulting in uniformpressure of the retainer 177, and a polished wafer 35 wherein a uniformlayer of silicon is removed. As with the previous embodiment, the gimbalpoint lies at or slightly below an interface of the wafer 35 and thework surface 37 on a side of the interface containing the turntable 27.This geometry maintains the work surface 37 and the retainer 177 inflatwise engagement with a uniform distance between the front surface 39and the work surface for more uniform pressure of the retainer. Thisconfiguration inhibits low pressure points from forming near thetrailing edge of the retainer 177 due to pivoting of the polishing head153 relative to the turntable 27, helping retain the wafer. Preferably,the lower spherical pivot 189 is formed from a high strength metal, suchas stainless steel, and the upper conical seat 187 is formed from aplastic material, such as PEEK, a polyaryletherketone resin, availablefrom Victrex USA Inc. of Westcheter, Pa., U.S.A. Both surfaces arehighly polished to inhibit wear debris generation and to minimizefriction within the spherical bearing assembly 159 and create a highlysmooth pivoting movement of the bearing assembly.

In the second embodiment, the semi-rigid connection 163 comprises aplurality of shoulder bolts 197 attached to the polishing head 153 (FIG.3). These shoulder bolts 197 extend upward from the polishing head 153and pass through a series of radial slots 199 in an annular flange 201extending laterally from the upper conical seat 187. The radial slots199 are sized slightly larger than the bolts 197 so that as the drivemechanism 45 rotates, the radial slots engage the shoulder bolts forinducing rotation of the polishing head 153. The additional clearancebetween the radial slots 199 and the bolts 197 allows the upper conicalseat 187 and the lower spherical pivot 189 to pivot slightly withrespect to one another and prevents the wafer 35 from falling out of thehead 153 and reduces wear on retainer 177. As with the previousembodiment, this pivoting allows for more uniform polishing andcontinuous transmission of rotation from the drive mechanism 45 to thepolishing head 153. The flange 201 and upper conical seat 187 are ofunitary, plastic construction. When the drive mechanism 45 is liftedupward after polishing, a bolt head 205 of each of the shoulder bolts197 engages the plastic flange 201, such that the polishing head 153 islifted from the work surface 37.

Applying polishing pressure through a membrane 169 has advantages over apolisher with using a rigid surface to support a wafer 35 duringpolishing. First, the head 153 can retain the wafer 35 without the useof an adhesive, reducing complexity and eliminating a possiblecontaminant. This embodiment secures the wafer 35 to the head 153 with avacuum, eliminating one source of potential contamination. Second,because the polishing pressure is applied to the wafer 35 via a membrane169, any particulate matter inadvertently caught between the wafer 35and the membrane 169 will not affect the polished surface. Withconventional systems, particulate matter, can become lodged between thewafer 35 and the rigid support surface (e.g., backing plate). Duringpolishing, this matter puts pressure on the back surface of the wafer,thereby pushing a small portion of the wafer outward toward thepolishing pad. The polishing operation seeks to flatten the wafer, andtypically flattens this small portion of the wafer pushed outward by theforeign matter. Once the wafer is removed from the rigid support, theportion of the wafer pushed out by the dust returns to its originalposition, leaving a dimple defect in the polished surface. With amembrane 169, any particulate matter lodged between the membrane and thewafer 35 will temporarily deform the membrane, not the wafer, allowingthe wafer to be polished normally without dimpling the wafer.

In operation, the wafer 35 and retainer ring 177 both engage the worksurface 37 (FIG. 3). As the polishing head 153 rotates, the membrane 169presses the wafer 35 into the work surface 37 while the ring 177 retainsthe wafer 35 within the head so that the friction between the worksurface and the wafer cannot pull the wafer out of the head. Theretainer 177 will wear slightly after extensive use, so that an offsetbetween a bottom 209 of the retainer and the membrane 169 may bemaintained. In effect, the ring 177 holds the polishing head 153 at theproper distance from the work surface 37 while the membrane 169 pressesthe wafer 35 into the work surface. By encircling the wafer 35 andextending downward from the polishing head 153 adjacent the wafer, theretainer 177 engages the wafer's edge 129 during polishing, even withsome wear of the retainer over time. As with the first embodiment, thepolishing head 153 and the turntable 27 rotate at different relativerotational speeds for more uniform polishing of the wafer 35. Thepolishing head 153 rotates at a rotational speed less that the turntable27. Preferably the drive mechanism 45 rotates the head 153 at arotational speed of between about forty percent (40%) and, about seventypercent (70%) of the rotational speed of the turntable 27. When thepolishing head 153 rotates at a rotational speed of about fifty-fivepercent (55%) of the rotational speed of the turntable 27, the polisherproduces optimally flat wafers.

Turning to a third embodiment of the polishing head, the presentembodiment comprises a polishing head 223 connected to the drivemechanism 45 for driven rotation of the polishing head (FIG. 4). Thepolishing head 223 is adapted to hold a wafer 35 for engaging a frontsurface 39 of the wafer 35 with a work surface 37 of the polishing pad.Like the previous embodiment, the present embodiment is directed toproviding uniform pressure over the wafer 35 for removal of a uniformlayer of silicon from a wafer made flat by a double-side polishingprocess or a fine grinding process.

A spherical bearing assembly, generally indicated at 227, connects thepolishing head 223 and the drive mechanism 45 for pivoting of thepolishing head. The spherical bearing assembly further comprises anupper conical seat 229 and a lower spherical pivot 231, similar to thesecond embodiment. The upper conical seat 229 is preferably welded tothe drive mechanism 45 along a distal end 232 of the drive mechanism,although other permanent forms of attachment are also contemplated aswithin the scope of the present invention. The polishing head 223 pivotsabout a gimbal point lying no higher than the work surface 37 when thepolishing head holds the wafer 35 in engagement with the polishing pad,thereby allowing the plane of the front surface 39 of the wafer tocontinuously align itself to equalize polishing pressure over the frontsurface of the wafer, while rotation of the polishing head is driven bythe driving mechanism 45. Preferably, as with the previous embodiments,the gimbal point lies below an interface of the wafer 35 and the worksurface 37 on a side of the interface containing the turntable 27 toequalize polishing pressure over the front surface 39 of the wafer. Auniform pressure is maintained between the front surface 39 and the worksurface 37 for more uniform polishing of the wafer by inhibitingpressure points from forming near the edge 129 of the wafer 35 due topivoting of the polishing head 223 relative to the turntable 27.

A semi-rigid connection, generally indicated at 233, between the drivemechanism 45 and the polishing head 223 transmits the rotational forceof the drive mechanism to the polishing head while permitting universalpivoting motion of the polishing head with respect to the drivemechanism. This connection 233 is similar to the semi-rigid connection163 of the second embodiment (FIG. 3) in that it uses shoulder bolts 235mounted on the polishing head 223 and passing through holes 237 in theupper conical seat 229. In contrast, however, the upper conical seat 229is not of unitary construction. The conical seat 229 includes a base 229a, welded to and extending laterally from the drive mechanism 45 toengage the shoulder bolts 235, while a portion 229 b of the upperconical seat 229 extends downwardly from the base to engage the lowerspherical pivot 231. The base 229 a is preferably formed from metal sothat it may be welded to the drive mechanism 45. The portion 229 b ispreferably formed from a plastic material, such as PEEK, apolyaryletherketone resin available from Victrex USA Inc. of Westcheter,Pa., U.S.A. Both the upper conical seat 229 and the lower sphericalpivot 231 are highly polished to inhibit wear debris generation and tominimize friction within the spherical bearing assembly 227 and create ahighly smooth pivoting movement of the bearing assembly.

An important distinction between the second and third embodiments is themethod of applying polishing pressure to the wafer 35. The thirdembodiment does not employ a membrane 169 but uses a rigid backing plate247 and a retainer 249, both attached to the polishing head 223, toretain the wafer 35. The backing plate 247 is flat and rigid, similar toa polishing block 115 of the first embodiment, being adapted to applyuniform pressure over the entire wafer 35 for even polishing of thewafer. Air pressure maintained within a cavity 251 formed between thepolishing head 223 and the backing plate 247 exerts downward force onthe backing plate and wafer 35. The retainer 249 extends downward fromthe polishing head 223 below the backing plate 247 for retaining thewafer 35 during polishing, similar to the second embodiment. The backingplate 247 moves independently of the retainer 249 so that as theretainer wears, the backing plate will extend outward a correspondinglysmaller distance for maintaining the same distance between the backingplate and retainer. This ensures that the proper engagement depth ismaintained between the retainer 249 and the wafer 35 for retaining thewafer within the retainer during polishing. When elevating the polishinghead 223 from the work surface 37, both before and after polishing, thedrive shaft 45 first lifts the spherical bearing assembly 227. A liftwasher 273 fits loosely over the drive mechanism 45 and the shoulderbolts 235 so that as the drive mechanism lifts the polishing head 223,the shoulder bolt heads 277 rest against the washer so that the drivemechanism can lift the polishing head. Without the lift washer 273, theheads 277 could pass through the holes 237, preventing lifting of thepolishing head from the work surface 37. The loose fit of the liftwasher 273 over the shoulder bolts 235 and drive mechanism 45 ensuresthat the washer does not impact the polishing process by inhibiting thegimbal action.

In operation, the third embodiment is virtually identical to theprevious two embodiments. This includes controlling the rotational speedof the drive mechanism 45 relative to the turntable 27. The same speedrange applies (between about forty percent (40%) and about seventypercent (70%)) and optimal rotational speed of about fifty-five percent(55%).

The present invention is additionally directed to a group of single sidepolished, monocrystalline semiconductor wafers 35 polished on a waferpolishing apparatus as described above in the first embodiment. Suchwafers 35 are typically held in a cassette, generally indicated at 253(FIG. 4A), for storage and transfer a plurality of wafers. Cassettes 253typically include a bottom portion 255, wafer slots 257 and a lid 259.After manufacture, a set of individual wafers 35 is typically loadedinto the cassette 253 for storage or shipping. These cassettes 253 canbe of various sizes for holding any number of wafers, such as 25, 20,15, 13, or 10 wafers per cassette. The wafers 35 are preferably formedfrom monocrystalline silicon, although the polishing apparatus andmethod of the present invention are readily adaptable to polishing othermaterials. The front surface 39 of a wafer 35 is polished to a finishpolish, while the back surface of the wafer is not polished to a finishpolish. Most wafers 35 additionally have a small chord of material, or anotch, removed from one edge 129 of the wafer, although the illustratedwafer exhibits no such chord.

The front surface 39 of the wafers 35 are uniformly flat for use inlithographic imprinting of circuits. Wafers 35 polished according to thepresent invention have a usable front surface 39 over an area from thecentral axis at least to within 2 millimeters (0.08 inches) of thecircumferential edge 129. Wafers are typically divided for analysis byprojecting a grid of sites onto the front surface 39, as shown in FIG.5. An outline of a semiconductor wafer 35 is shown. Any predeterminednumber, geometrical size or geometrical shape of sites may be overlainon the front surface 39 of the wafer, depending upon the wafer'sapplication. Most commonly the sites are squares or rectangles ofuniform size and shape. Some sites are categorized as whole sites 261and others as partial sites 263. For the present analysis, amultiplicity of whole sites 261 lie entirely within the front surface 39of the wafer 35 and a multiplicity of partial sites 263 lie partially onthe front surface and partially outside the circumferential edge 129 ofthe wafer. When polished according to the present invention, theflatness of the partial sites 263 is substantially the same as theflatness of the whole sites 261. For purposes of discussion, thefollowing data analysis is based upon semiconductor wafers 35 having adiameter of approximately 200 millimeters (7.9 inches) with a projectedgrid of twenty partial sites 263 and thirty-two whole sites 261, asshown in FIG. 5. The wafers used in this analysis were not selectedaccording to their flatness, but represent a typical production groupingof wafers. Each site is preferably square in shape, measuring 25millimeters (0.98 inches) along each side thereof. This corresponds toan area of each whole site 261 or each partial site 263 of about twopercent (2%) of the area of the front surface 39 of the wafer. Thepartial sites 263 situated near the wafer's 35 edge 129 additionallycomprise an outer ring of sites that are subject to improvement due tothe present invention. Although the data analysis is based uponmeasurements from 200 millimeter (7.9 inch) wafers, the presentinvention is readily applicable to wafers of other diameters, such as100 millimeter (3.9 inch), 150 millimeter (5.9 inch) and 300 millimeter(12 inch) wafers, to name a few.

Single side polished wafers 35 polished according to the presentinvention will exhibit partial sites 263 with uniform flatnesssubstantially similar to the whole sites 261. This is a substantialimprovement over single side polished wafers 35 polished on conventionalpolishers which often exhibit unacceptable roll-off near the edge 129 ofthe wafer. The front surface 39 of the wafer 35 of the present inventionis a highly polished surface that is uniformly flat across the majorityof the front surface, including a wafer surface area up to within about2 millimeters (0.08 inches) of the wafer's circumferential edge 129.Typically, roll-off degrades the flatness of the wafer's 35 edge 129enough to make the wafer usable for lithographic processing from acentral axis to within 3 millimeters (0.12 inches) of the wafer's edge.Broadening a wafer's 35 usable area from 3 millimeters (0.12 inches) towithin 2 millimeters (0.08 inches) of the wafer's 35 edge 129 increasesthe usable wafer area by two percent (2%). It is believed that theusable area extends closer to the edge than 2 millimeters (0.08 inches).More importantly, the partial sites 263 near the wafer's 35 edge 129exhibit better flatness characteristics, so that lithography of thesepartial sites is more likely to create an accurate lithograph on thewafer. Better focused edge lithography yields fewer edge defects,translating into fewer device failures. Moreover, wafers 35 of thepresent invention are more symmetrical about the circumference of thewafer. More symmetrical wafers 35 are beneficial because they allow foruniform processing of all portions of a wafer.

For example, FIG. 6 depicts a population of 200 millimeter (7.9 inch)diameter wafers polished on a conventional single-side polisher having agimbal point approximately 51 millimeters (2.0 inches) above the worksurface. The data was processed with a 2.0 millimeter (0.079 inch) edgeexclusion, partials active and dimples included. In addition, sites 25millimeters (0.98 inches) square were used to gather and interpret theflatness data. Particular wafers were dropped from the data set asunacceptable for sale, and thus for analysis, if any single site on awafer had an SFQR value of greater than 0.250 microns (9.84microinches). These wafers 35 are presumed to exhibit dimpling defects.Of the original 363 wafers in the sample, 15 were dropped, leaving 348wafers and 348 data points. These data closely conform to historicalperformance of conventional single-side wafer polishers. A single datapoint is plotted from each wafer representing the largest SFQR value forany partial site 263 on the wafer, as measured with an industry-standardcapacitance tool, rather than an emerging technology optical tool. Forexample, the data disclosed herein were gathered with an Ultrascan 9000Series (e.g., Ultrascan 9600) manufactured by ADE Corporation ofWestwood, Mass. These data points are plotted in FIG. 6 and yield anaverage of 0.136 microns (5.34 microinches) for the largest SFQR partialsite 263 over the entire population of wafers. To compare thisconventionally polished population to the present invention, FIG. 7depicts a population of wafers polished on a wafer polisher of thepresent invention having a driven polishing head and a gimbal point atthe interface of the front surface 39 of the wafer and the work surface.The wafers 35 were of the same size and processed in the same way exceptfor the polishing step. Of the original 1745 wafers in the sample, 86wafers were dropped for having any site on a wafer with an SFQR value ofgreater than 0.250 microns (9.84 microinches), again presumably becauseof dimpling, leaving 1659 wafers and 1659 data points. These data yielda smaller population average of 0.102 microns (4.02 microinches), animprovement of 24.8 percent (24.8%) over the conventional process.Therefore, a wafer polished according to the present invention shouldyield a maximum partial site 263 SFQR on average of less than about0.105 microns (4.13 microinches). These wafers exhibiting improvedflatness allow for accurate lithography of substantially the entirefront surface 39 of the wafer.

Another measurement of edge flatness is the average of the SFQR valuesfor all partial sites 263 on a wafer. FIG. 8 depicts this measure forthe same population of conventionally polished wafers shown in FIG. 6,and the average of the SFQR values for all partial sites 263 on a waferon average is 0.088 microns (3.46 microinches). FIG. 9 depicts theidentical measure for the same population of wafers polished accordingto the present invention shown in FIG. 7, wherein the average of theSFQR values for all partial sites 263 on a wafer on average is 0.064microns (2.54 microinches). Wafers polished with an apparatus or methodof the present invention yield a 26.7 percent (26.7%) increase inflatness over the conventional process. The wafers 35 exhibit improvedflatness, allowing for accurate lithography of substantially the entirefront surface 39 of the wafer.

An additional flatness parameter of importance is the flatnesscharacteristics of adjacent sites. Lithography requires careful focusingof a lithography machine on the surface of a wafer. Focusing on wholesites 261 is routine, but focusing on partial sites 263 requires moreadvanced techniques, which add cost and time to the lithography process.Therefore, wafer lithographers often focus their lithographers on afocusing whole site 267,267′ and then move to an immediately adjacentnon-focusing partial site 269, assuming that the two sites are polishedto a similar flatness so that the lithography of the partial site willalso be in focus. These focusing whole sites 267,267′ and non-focusingpartial sites 269, although identical to the previous whole sites 261and partial sites 263, are renumbered here to further describe themovements of a lithographer. FIG. 10 depicts (by arrows) the lithographymachine movements from focusing sites 267,267′ to non-focusing sites269. For instance, a lithographer would likely not be able to accuratelyfocus on site X because it is a partial site 269. Therefore, thelithographer would typically focus on site Y, and then move the camerain the direction indicated by the arrow to perform lithography on siteX. The assumption regarding similar flatness characteristics of adjacentsites is only valid where the non-focusing sites 267 are polishedsimilarly to the focusing sites 269. Where a wafer exhibits large edgeroll-off, however, this assumption may lead to lithography errors. Awafer with comparable flatness characteristics in the center and at theedges 129 renders this assumption more acceptable.

To quantify whether a wafer exhibits similar polishing at the partialsites and at an adjacent inner ring of whole sites, flatness data forthe outer ring of non-focusing partial sites 269 and an inner ring offocusing whole sites 267 as defined in FIG. 5 (sites 267′ are notincluded in the data for the focusing whole sites 267) can be compared.The data shown in FIG. 11 depict the difference between the average ofthe SFQR values for each site of an outer ring of twenty non-focusingpartial sites 269 and an average of the SFQR values for each site of animmediately adjacent inner ring of sixteen focusing whole sites 267, forthe same population of conventionally polished wafers shown in FIG. 6.The average SFQR difference for wafers polished on a conventionalpolisher is 0.030 microns (1.2 microinches). The data shown in FIG. 12depict the difference between the average of the SFQR values for eachsite of an outer ring of twenty partial sites 269 and an average of theSFQR values for each site of an immediately adjacent inner ring ofsixteen whole sites 267, for the same population of wafers of thepresent invention shown in FIG. 7. The average SFQR difference forwafers of the present invention is 0.013 microns (0.52 microinches).Wafers polished with an apparatus or method of the present inventionyield a fifty-five percent (55%) increase in adjacent-site flatness overthe conventional process. Wafers of the present invention allow foraccurate lithography of partial sites 269 without refocusing thelithography apparatus on each partial site.

Reviewing the data in another way, FIG. 13 depicts the percentiledifference between the average of the SFQR values for each site of theouter ring of partial sites 269 and the average of the SFQR values foreach site of the immediately adjacent inner ring of whole sites 267 fora conventional wafer polisher. The average percentile difference betweenthe outer and inner ring average SFQR for wafers 35 polished on aconventional polisher is 56.3 percent (56.3%), for the same populationof conventionally polished wafers shown in FIG. 6. In contrast, FIG. 14depicts the same percentile difference for the same population of wafers35 of the present invention used to construct FIG. 7. The averagepercentile difference between the outer and inner ring average SFQR forwafers polished on a wafer polisher of the present invention is 18.3percent (18.3%). Thus, the polisher of the present invention yields a67.6 percent (67.6%) decrease in this parameter over a conventionalpolisher. Therefore, wafers 35 polished according to the presentinvention will yield results having an average SFQR for an outer ring ofpartial sites 269 of less than fifty-five percent (55%) larger than anaverage of the SFQR values for an the inner ring of whole sites.Moreover, the present invention will yield an average SFQR differencebetween an inner ring and outer ring of less than thirty percent (30%)and likely less than eighteen percent (18%).

One final measure of wafer 35 flatness is the percentile differencebetween a maximum SFQR value for any partial site 263 of each wafer anda maximum SFQR value for any whole site 261 of each wafer (FIG. 5).Referring now to FIG. 15, data showing such a comparison yields anaverage percentile difference between a partial site 263 maximum SFQRand a whole site 261 maximum SFQR of 21.2 percent (21.2%), for the samepopulation of conventionally polished wafers 35 shown in FIG. 6. Incontrast, FIG. 16 depicts the same percentile difference for the samepopulation of wafers 35 of the present invention used to construct FIG.7. The average percentile difference between the partial site 263maximum SFQR and the whole site 261 maximum SFQR for wafers 35 polishedon a wafer polisher of the present invention is −10.7 percent (−10.7%).The negative percentile value indicates that for wafers 35 polishedaccording to the present invention, the partial site 263 maximum SFQR islikely less than the whole site 261 maximum SFQR. Contrary toconventionally polished wafers 35, these wafers tend to have lower SFQRmaximums in their partial sites 263, rather than in their whole sites261. Thus, the polisher of the present invention yields a significantimprovement in this parameter over a conventional polisher. Therefore,wafers polished according to the present invention will yield maximumSFQR values for partial sites 263 which are no more than twenty percent(20%) larger than the maximum SFQR values for the whole sites 261.Moreover, the present invention will yield average maximum SFQR valuesfor partial sites 263 of about the same and likely ten percent (10%)less than the maximum SFQR values for the whole sites 261.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

1. Wafer polishing apparatus comprising: a base for supporting elementsof the polishing apparatus; a turntable having a polishing pad thereonand mounted on the base for rotation of the turntable and polishing padrelative to the base about an axis perpendicular to the turntable andpolishing pad, the polishing pad including a work surface engageablewith a front surface of a wafer for polishing the front surface of thewafer; a drive mechanism mounted on the base for imparting rotationalmotion about an axis substantially parallel to the axis of theturntable; a polishing head connected to the drive mechanism for drivenrotation of the polishing head, the polishing head being adapted to holdat least one wafer for engaging a front surface of the wafer with thework surface of the polishing pad; and a spherical bearing assemblymounting the polishing head on the drive mechanism for pivoting of thepolishing head about a gimbal point lying below the interface of thefront surface of the wafer and the work surface on a side of theinterface containing the turntable when the polishing head holds thewafer in engagement with the polishing pad, thereby allowing the planeof the front surface of the wafer to continuously align itself to applyslightly more polishing pressure to a trailing edge of the wafer, whilerotation of the polishing head is driven by the driving mechanism formaintaining the front surface and work surface in flatwise engagementfor more uniform polishing of a semiconductor wafer.
 2. Wafer polishingapparatus as set forth in claim 1 further comprising a semi-rigidconnection between the drive mechanism and the polishing head forimparting a rotational force from the drive mechanism to the polishinghead so that the polishing head and drive mechanism rotate conjointly,while permitting universal pivoting motion of the polishing head withrespect to the drive mechanism about the spherical bearing assembly. 3.Wafer polishing apparatus as set forth in claim 2 wherein the drivemechanism is adapted to drive the wafer carrier at a rotational speed ofbetween about forty percent (40%) and about seventy percent (70%) of therotational speed of the turntable.
 4. Wafer polishing apparatus as setforth in claim 3 wherein the drive mechanism is adapted to drive thewafer carrier at a rotational speed of about fifty-five percent (55%) ofthe rotational speed of the turntable.
 5. Wafer polishing apparatus asset forth in claim 2 wherein the semi-rigid connection comprises aflexible connection between the drive mechanism and the polishing head.6. Wafer polishing apparatus as set forth in claim 5 wherein theflexible connection further comprises a torque transmittal boot attachedto the drive mechanism and the polishing head, thereby allowing thepolishing head to pivot with respect to the drive mechanism about thespherical bearing assembly for transmitting rotation from the drivemechanism to the polishing head.
 7. Wafer polishing apparatus as setforth in claim 6 wherein the torque transmittal boot is made of anelastomeric material having a stiffness for transmitting the rotationalenergy of the drive mechanism to the polishing head and a resiliency toallow pivoting movement of the polishing head.
 8. Wafer polishingapparatus as set forth in claim 7 wherein the elastomeric material isrubber.
 9. Wafer polishing apparatus as set forth in claim 8 whereinsaid spherical bearing assembly further comprises an upper bearingmember, a lower bearing member and a plurality of ball bearings, saidball bearings being engageable with the upper bearing member and thelower bearing member for relative movement between the members so thatthe polishing head may pivot relative to the drive mechanism.
 10. Waferpolishing apparatus comprising a base for supporting elements of thepolishing apparatus; a turntable having a polishing pad thereon andmounted on the base for rotation of the turntable and polishing padrelative to the base about an axis perpendicular to the turntable andpolishing pad, the polishing pad including a work surface engageablewith a front surface of a wafer for polishing the front surface of thewafer; a drive mechanism mounted on the base for imparting rotationalmotion about an axis substantially parallel to the axis of theturntable; a polishing head connected to the drive mechanism for drivenrotation of the polishing head, the polishing head being adapted to holdat least one wafer for engaging a front surface of the wafer with thework surface of the polishing pad; a spherical bearing assembly mountingthe polishing head on the drive mechanism for pivoting of the polishinghead about a gimbal point lying below the interface of the front surfaceof the wafer and the work surface on a side of the interface containingthe turntable when the polishing head holds the wafer in engagement withthe polishing pad, thereby allowing the plane of the front surface ofthe wafer to continuously align itself to apply slightly more polishingpressure to a trailing edge of the wafer, while rotation of thepolishing head is driven by the driving mechanism for maintaining thefront surface and work surface in flatwise engagement for more uniformpolishing of a semiconductor wafer, said spherical bearing assemblycomprising an upper bearing member, a lower bearing member, and aplurality of ball bearings, said ball bearings being engageable with theupper bearing member and the lower bearing member for relative movementbetween the members so that the polishing head may pivot relative to thedrive mechanism, wherein the upper bearing member and the lower bearingmember have spherically shaped bearing surfaces, wherein the center ofeach spherical bearing surface corresponds to the gimbal point and anyline normal to either surface passes through the gimbal point.
 11. Waferpolishing apparatus as set forth in claim 10 wherein the drive mechanismfurther comprises a motor and a gearbox mounted on the base and attachedto the drive mechanism for rotation of the drive mechanism.
 12. Waferpolishing apparatus as set forth in claim 11 wherein an axis of rotationof the polishing head is spaced apart from an axis of rotation of theturntable.
 13. Wafer polishing apparatus comprising: a base forsupporting elements of the polishing apparatus; a turntable having apolishing pad thereon and mounted on the base for rotation of theturntable and polishing pad relative to the base about an axisperpendicular to the turntable and polishing pad, the polishing padincluding a work surface engageable with a front surface of a wafer forpolishing the front surface of the wafer; a drive mechanism mounted onthe base for imparting rotational motion about an axis substantiallyparallel to the axis of the turntable; a polishing head connected to thedrive mechanism for driven rotation of the polishing head, the polishinghead being adapted to hold at least one wafer for engaging a frontsurface of the wafer with the work surface of the polishing pad aspherical bearing assembly mounting the polishing head on the drivemechanism for pivoting of the polishing head about a gimbal point lyingbelow the interface of the front surface of the wafer and the worksurface on a side of the interface containing the turntable when thepolishing head holds the wafer in engagement with the polishing pad,thereby allowing the plane of the front surface of the wafer tocontinuously align itself to apply slightly more polishing pressure to atrailing edge of the wafer, while rotation of the polishing head isdriven by the driving mechanism for maintaining the front surface andwork surface in flatwise engagement for more uniform polishing of asemiconductor wafer; and a semi-rigid connection between the drivemechanism and the polishing head for imparting a rotational force fromthe drive mechanism to the polishing head so that the polishing head anddrive mechanism rotate conjointly, while permitting universal pivotingmotion of the polishing head with respect to the drive mechanism aboutthe spherical bearing assembly, wherein the semi-rigid connectioncomprises at least one shoulder bolt attached to the polishing head andpassing through at least one radial slot in the drive mechanism, theradial slot being sized slightly larger than the bolt so that as thedrive mechanism rotates, the radial slot is engageable with the shoulderbolt for inducing rotation of the polishing head, while allowing thespherical bearing assembly to pivot slightly for more uniform polishingand continuous transmission of rotation from the drive mechanism to thepolishing head.
 14. Wafer polishing apparatus as set forth in claim 13further comprising a membrane mounted on the polishing head, saidmembrane having an outer surface engageable with a wafer for mountingthe wafer to the polishing head and an inner surface opposite the outersurface facing the polishing head.
 15. Wafer polishing apparatus as setforth in claim 14 further comprising a vacuum source in fluidcommunication with a cavity formed between the inner surface of themembrane and the polishing head, said membrane having at least one holeformed therein so that when a vacuum is drawn in the cavity, themembrane can draw the wafer up against the membrane and hold the wafer,said membrane further holds the wafer when the wafer engages the worksurface, whereby air may then be directed into the cavity, eliminatingthe vacuum and providing uniform air pressure within the cavity forpressing the wafer surface uniformly against the work surface.
 16. Waferpolishing apparatus as set forth in claim 15 further comprising aretainer attached to the polishing head, said retainer extending fromthe polishing head below the wafer and membrane for retaining the waferduring polishing.
 17. Wafer polishing apparatus as set forth in claim 16wherein the membrane is movable independently of the retainer so that asthe retainer wears, an offset between a bottom of the retainer and themembrane may be maintained.
 18. Wafer polishing apparatus as set forthin claim 17 wherein the retainer is ring-shaped for encircling themembrane and wafer to retain the wafer during polishing.
 19. Waferpolishing apparatus as set forth in claim 18 wherein the sphericalbearing assembly further comprises an upper conical seat attached to androtated with the drive mechanism and a lower spherical pivot rigidlymountable on the polishing head, said lower spherical pivot isengageable with the upper conical seat for pivotable movement of thepolishing head with respect to the drive mechanism.
 20. Wafer polishingapparatus as set forth in claim 19 wherein the lower spherical pivot hasan upwardly directed spherical face, wherein any line normal to thespherical face passes through the gimbal point.
 21. Wafer polishingapparatus as set forth in claim 13 further comprising a rigid backingplate and a retainer, both attached to the polishing head, said backingplate being adapted to apply uniform pressure over the entire wafersurface for even polishing of the wafer and said retainer extending fromthe polishing head below the backing surface for retaining the waferduring polishing.
 22. Wafer polishing apparatus as set forth in claim 21wherein the backing plate is movable independently of the retainer sothat as the retainer wears, an offset between a bottom of the retainerand the backing plate may be maintained.
 23. Wafer polishing apparatusas set forth in claim 22 wherein the retainer is ring-shaped forencircling the backing plate and wafer to retain the wafer duringpolishing.
 24. Wafer polishing apparatus as set forth in claim 23wherein the polishing head is adapted to hold a single wafer forengaging the front surface of the wafer with the work surface of thepolishing pad.
 25. A method of polishing a semiconductor wafercomprising the steps of: placing the semiconductor wafer in a polishinghead of a wafer polishing apparatus; driving rotation of a polishing padon a turntable of the polishing apparatus about a first axis; drivingrotation of the polishing head generally about a second axisnon-coincident with the first axis; positioning the wafer held by thepolishing head so that a front surface of the wafer engages a worksurface of the polishing pad; urging the front surface of the waferagainst the polishing pad; holding the polishing head for free pivotingmovement about a gimbal point located below an interface of the worksurface and the front surface of the wafer on a side of the interfacecontaining the turntable as rotation of the polishing head continues tobe driven to apply slightly more polishing pressure to a trailing edgeof the wafer in response to a net force about the gimbal point acting ina direction perpendicular to the front surface of the wafer, whilepreventing pivoting of the front surface of the wafer under forcesparallel to the front surface of the wafer passing generally through thegimbal point; disengaging the wafer from the turntable; and removing thewafer from the polishing head.
 26. A method as set forth in claim 25wherein the step for placing the semiconductor wafer comprises adheringthe wafer to a polishing block and securing the polishing block to thepolishing head.
 27. A method as set forth in claim 25 wherein thedriving step comprises rotating the polishing head at a speed less thanthe rotational speed of the turntable.
 28. A method as set forth inclaim 27 wherein the driving step comprises rotating the drive mechanismat a speed of between about forty percent (40%) and about seventypercent (70%) of the rotational speed of the turntable.
 29. A method asset forth in claim 28 wherein the driving step comprises rotating thedrive mechanism at a speed of about fifty-five percent (55%) of therotational speed of the turntable.
 30. A method as set forth in claim 25wherein the placing step further comprises mounting the wafer on amembrane mounted on the polishing head by evacuating a cavity behind themembrane to draw the wafer up against the membrane and hold the waferduring the polishing step, the method further comprising selectivelyvarying air pressure within the cavity for pressing the wafer surfaceuniformly against the work surface.